This invention relates to underground retorting of oil shale, and more particularly, to a process of forming an in situ oil shale retort.
Researchers have now renewed their efforts to find alternative sources of energy and hydrocarbons in view of recent rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal and tar sand by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah, and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons, can be done in surface retorts above ground or in situ retorts underground. In situ retorts require less mining and handling than surface retorts.
In in situ retorts, a flame front is continuously passed downward through a bed of oil shale to liberate shale oil, off gases and oil shale retort water. There are two types of in situ retorts: true in situ retorts and modified in situ retorts. In true in situ retorts, all of the oil shale is retorted underground as is, without mining or transporting any of the shale to above ground locations. The shale can be explosively rubblized, if desired. In modified in situ retorts, some of the oil shale is mined and conveyed to the surface to create an underground cavity or void space in the retorting area. The remaining underground oil shale is then explosively rubblized to substantially fill the void space. The oil shale which has been conveyed to the surface is retorted above ground.
Over the years various methods have been suggested for explosively forming and retorting in situ oil shale retorts. Typifying these methods and other methods of underground mining are those found in U.S. Pat. Nos. 3,762,771; 3,980,339; 4,043,595; 4,043,596; 4,043,597; 4,043,598; 4,146,272; 4,175,490; 4,192,553; 4,192,554; 4,201,419; 4,205,610; 4,210,366; 4,245,865; 4,262,965; 4,272,127; and Canadian Pat. No. 1,012,564. These prior art methods have met with varying degrees of success.
In forming in situ retorts, explosion gases are conventionally vented through the bottom or, alternatively through a vertical wall of the retort. The explosion gases are constrained by the roof and other walls of the retort, and are deflected back through crevices in the rubblized mass and along the walls of the retort before being vented. Such deflection causes undesirable backflow, turbulence and crevice expansion, which create enlarged vertical, horizontal and irregular channels throughout the rubblized bed and along the retort walls. As a result, during retorting, hot gases flow down these channels and often bypass large portions of the bed, leaving significant portions of the rubblized shale unretorted.
Channelling also causes many deleterious effects including tilted (nonhorizontal) and irregular flame fronts in close proximity to the retorting zone, as well as fingering, that is, flame front projections which extend downward into the raw oil shale far ahead of other portions of the flame front. Irregular and tilted flame fronts can lead to flame front or oxygen breakthrough, incomplete retorting and burning of the product shale oil. Flame fronts in close proximity to the advancing front of the retorting zone can also cause combustion of product shale oil. If a narrow portion of the flame front advances completely through the retorting zone, it can ignite the effluent oil and off gases and may cause explosions. It has been estimated that losses from burning in in situ retorting are as high as 40% of the product shale oil. Fingering can cause coking and thermal cracking of the liberated shale oil.
It is therefore desirable to provide an improved process, which overcomes most, if not all, of the preceding problems.